1. Field of the Invention
The present invention relates to a storage system which exchanges data with a higher-level device, and a snapshot data preparation method in this storage system.
2. Description of the Related Art
Conventionally, in information processing systems comprising a storage system, a proposal has been made with the object of reducing the load on host devices relating to snapshot data control, and increasing the speed of the storage system. In this proposal, when a host device desires the preparation of snapshot data, this host device transmits an instruction indicating a snapshot data preparation request to a storage system, the control part (disk control device) of the storage system reads out user data from a user data region in the disk driving device of the storage system, and this read-out user data is stored in a snapshot data region in the disk driving device, so that snapshot data is prepared. As a result, the burden of snapshot data preparation is dispersed between the host device and the storage system. Furthermore, the amount of data that is exchanged between the host device and the storage system for the purpose of snapshot data preparation can be reduced (for example, see Japanese Patent Application Laid-Open No. 2004-118413).
Known methods of snapshot data preparation include a system called a “physical snapshot” (so-called shadow image) and a system called a “logical snapshot” (so-called quick shadow). In the logical snapshot system, a pool region which is a data space used to save data prior to updating among the original data stored in the primary logical volume is set in the disk drive device. Furthermore, from the point in time at which backup of the primary (logical) volume is initiated, original data prior to updating is stored in the pool region only for the storage region in which data updating of the primary (logical) volume by the writing of new data or the like is performed. In this state, if there is a read request for snapshot data from the host device, the disk control device prepares the data at the backup starting point by synthesizing the data stored in the pool region and the original data that has not been updated in the primary (logical) volume, and transfers this data to the host device.
In the logical snapshot system, only original data prior to updating among the original data stored in the primary (logical) volume is stored in the pool region; accordingly, the following advantage is obtained: namely, if the amount of updated data is small, the amount of the pool region that is used is also small. However, when the amount of updated data is large, the amount of the pool region that is used is also unavoidably increased to a corresponding extent, and especially when a plurality of pairs of primary (logical) volumes and secondary (logical) volumes are formed, and snapshot data of these volumes of the like is prepared, it is difficult to predict the extent of future pool region use. In some cases, furthermore, the pool region may become filled with saved data from the primary (logical) volume, so that the snapshot data preparation processing ends abnormally.